Extending ligand field molecular mechanics to modelling organometallic π-bonded systems: applications to ruthenium-arenes.

The ligand field molecular mechanics method has been extended to treat η(6)-arene ligands coordinated to a ruthenium(II) centre by employing a dummy atom located at the centroid of the arene ring and distributing the forces on the dummy to the arene carbon atoms. Angular overlap model parameters based on orbital energies derived from Kohn-Sham density functional theory (KS-DFT) calculations show that, relative to the Ru-dummy vector, the arene behaves as a very strong π donor and weak σ donor. Based on KS-DFT geometries, partial atomic charges and potential energy scans for a series of homoleptic and half sandwich complexes spanning arene, am(m)ine, imine, pyridyl, hydride and chloride ligands, a new LFMM force field has been developed which accurately reproduces the KS-DFT data. This FF was validated against 47 half-sandwich complexes obtained from the Cambridge Structural Database which, after minor corrections to account for the systematic errors between our chosen functional (BP86) and the experimental structures, yields a 'structurally tuned' FF where 93% of the Ru-L contacts are reproduced to 0.05 Å or better and all bar two bond lengths are within 0.1 Å of experiment. Over half the systems have non-hydrogen-atom rmsds of less than 0.5 Å. Larger differences are usually due to rotation of the arene moiety which is shown by ligand field molecular dynamics (LFMD) simulations to be an inherently low-energy process. Comparisons between LFMD and Car-Parrinello MD for [Ru(p-cymene)(ethylenediamine)Cl](+)show that LFMD is equally accurate but much faster enabling modelling of dynamic properties which occur on a timescale beyond the scope of CPMD.